A wide variety of automatic, body-implantable medical devices for performing various therapeutic and diagnostic functions are known in the prior art. A common example, the implantable cardiac pacemaker, functions to deliver electrical stimulating pulses to a patient's heart. Early pacemakers, which typically included discrete analog components, delivered stimulating pulses at a fixed periodic rate without regard to the presence or absence of naturally occurring heartbeats. Later, so-called demand pacemakers, which were capable of sensing intrinsic cardiac activity and delivering stimulating pulses only when necessary, were developed. Modern pacemakers typically utilize digital circuitry of considerable complexity, and are thus vastly more sophisticated in their operational capabilities. The increased capabilities of state-of-the-art pacemakers has expanded the operating modalities to encompass both chambers of the heart, and has also led to the addition of features such as multiprogrammability.
As the functional sophistication and complexity of implantable devices such as pacemakers, defibrillators, cardioverters and the like has increased over the years, it has been increasingly more important for such devices to be equipped with a telemetry system for facilitating communication of information between the implanted device and an external programming and/or control unit.
For example, even in connection with the earliest fixed-rate, non-inhibited pacemakers, it was apparent that it would be desirable for a physician to noninvasively exercise at least some amount of control over the device, e.g., to turn the device on or off, or to adjust the fixed pacing rate, after implant. In early devices, one way the physician was able to have some control over implantable device operation was through the provision of a magnetic reed switch in the implantable device. After implant, the reed switch would be actuated (closed) by placing a magnet over the implant site. Reed switch closure could then be used, for example, to alternately activate and deactivate the device. Alternatively, the fixed pacing rate of the device could be adjusted up or down by incremental amounts based upon the duration of the reed switch closure interval. Many different schemes utilizing a reed switch to adjust operational parameters of medical devices have been developed. See, for example, U.S. Pat. No. 3,311,111 to Bowers; U.S. Pat. No. 3,518,997 to Sessions; U.S. Pat. No. 3,768,486 to Berkovits; U.S. Pat. No. 3,631,860 to Lopin; U.S. Pat. No. 3,738,369 to Adams et al., U.S. Pat. No. 3,805,796 to Terry, Jr.; and U.S. Pat. No. 4,066,086 to Alferness et al.
The need to communicate more and more information to implanted devices with increasing levels of functionality quickly rendered the simple reed switch closure arrangement insufficient. Also, it has become apparent that it is desirable not only to allow information to be communicated to the implanted device from an external unit (referred to as "downlink telemetry"), but also to enable the implanted device to communicate information to the external unit (referred to as "uplink telemetry").
Uplink telemetry capabilities in an implantable device system are particularly advantageous with state-of-the-art pacemakers which interact with the heart in a complex fashion. Even relatively simple diagnostic tasks, such as verifying proper operation of the pacemaker, can be difficult with sophisticated, state-of-the art pacemakers, whose pacing algorithms involve, for example, automatically varying timed responses to sensed inputs. In addition, a pacemaker's pacing algorithm may be such that the pacemaker's responses may depend upon prior events, which means that interpretation of responses also requires information about what events have been sensed by the device and how such events are interpreted by the device.
Traditionally, verification of pacemaker operation was less difficult, and was accomplished with the aid of an electrocardiogram (ECG), which shows electrical cardiac activity detected either on the skin surface of the patient (surface ECG), or from an implanted sensing lead (intracardiac ECG). As will be appreciated by those of ordinary skill in the art, the ECG displays the physiological waveform of the heart as a complex periodic waveform with P, Q, R, S, and T portions. Pacemaker stimulating pulses appear as narrow pacemaker artifacts on an ECG trace. By noting the relationship between the pacemaker artifacts and various elements of the physiological waveform, the physician can analyze the operating characteristics of the pacemaker to verify its proper and safe performance.
Those of ordinary skill in the art will further appreciate, however, that modem dual-chamber pacemakers, with complex pacing algorithms and features such as rate- or activity-responsiveness, multiple chamber sensing and pacing, multiprogrammability, multi-modal operation, and the like, have responses to physiological events which may be difficult to analyze based solely upon the observations of a patient's electrocardiogram. Consequently, there has been a need to provide additional uplink telemetry information to the attending physician or clinician to simplify the analysis of pacemaker operation.
One prior art technique which is directed to the above-described problem is presented in U.S. Pat. No. 3,662,759 to Dabolt. According to the Dabolt '759 patent, a narrow sub-threshold pulse is applied to the heart via the pacing lead system each time the demand pacemaker escape interval is reset by sensed spontaneous cardiac activity. This sub-threshold pulse is insufficient to stimulate the heart, but its steep rise time generates sufficient radio-frequency harmonics to be detected by a conventional radio receiver. In operation, a radio "click" is produced each time a naturally occurring R-wave is detected and used by the pacemaker circuitry to reset the escape interval of the pacemaker. Thus, according to the Dabolt '759 patent, proper sensing by the pacemaker's sensing circuitry can be verified. Although the system disclosed in the Dabolt '759 patent provides a convenient method of producing a remote indication of a sensed event with a minimum of equipment, no permanent record of the sensed event is produced by this technique, nor is the system applicable to the analysis of operation of more complex pacemakers. In addition, the delivery of sub-threshold pulses in accordance with the Dabolt '759 patent may result in earlier depletion of the implanted device's power supply.
Prior art uplink telemetry systems are known in which sensed diagnostic data or patient data is transmitted, via an uplink telemetry channel, to an external apparatus for display. For example, it is known to provide a pacemaker with a separate analog uplink telemetry channel for transmission of real-time intracardiac ECG data, the analog transmission channel being separate from and in addition to the downlink channel for operator programming and interrogation. While, of course, multiple channels can be used if there is no limit on expense, size, or power requirements, there is a great need for efficient use of the available communications channel, in order to handle the normal programming and interrogation requirements, as well as the transmission of patient and other diagnostic data.
Known pacemaker systems have accordingly been provided with what is referred to as Marker Channel.TM. functionality, in which uplink information regarding the pacemaker's operation and the occurrence of physiological events is communicated to an external unit. The Marker Channel.TM. information can then be printed or displayed in relation to an ECG so as to provide supplemental information regarding pacemaker operation. For example, events such as pacing or sensing of natural heartbeats are recorded with a mark indicating the time of the event relative to the ECG. This is helpful to the physician in interpreting the ECG, and in verifying proper operation of the pacemaker. One example of a Marker Channel.TM. system is disclosed in U.S. Pat. No. 4,374,382 to Markowitz, entitled "Marker Channel Telemetry System for a Medical Device." The Markowitz '382 patent is hereby incorporated by reference herein in its entirety.
Existing systems which provide a Marker Channel.TM. output operate basically by outputting an indication of a physiological or pacemaker event, e.g., a delivered stimulating pulse or a sensed heartbeat, at about the time of the event, thereby inherently providing the timing of the event in relation to the recorded ECG. Alternatively, the Marker Channel.TM. system can accumulate data over a period of time, e.g., one cardiac cycle, and transmit a batch of data for that interval at the beginning of the next interval. This is what appears to be proposed in U.S. Pat. No. 4,601,291 to Boute et al., entitled "Biomedical System with Improved Marker Channel Means and Method." The Boute et al. '291 patent is also incorporated by reference herein in its entirety.
Various telemetry systems for providing the necessary communications channels between an external unit and an implanted device have been shown in the art. Telemetry systems are disclosed, for example, in the following U.S. Pat. Nos.: U.S. Pat. No. 4,539,992 to Calfee et al. entitled "Method and Apparatus for Communicating With Implanted Body Function Stimulator"; U.S. Pat. No. 4,550,732 to Batty Jr. et al. entitled "System and Process for Enabling a Predefined Function Within An Implanted Device"; U.S. Pat. No. 4,571,589 to Slocum et al. entitled "Biomedical Implant With High Speed, Low Power Two-Way Telemetry"; U.S. Pat. No. 4,676,248 to Berntson entitled "Circuit for Controlling a Receiver in an Implanted Device"; U.S. Pat. No. 5,127,404 to Wyborny et al. entitled "Telemetry Format for Implanted Medical Device"; U.S. Pat. No. 4,211,235 to Keller, Jr. et al. entitled "Programmer for Implanted Device"; and U.S. Pat. No. 4,556,063 to Thompson et al. entitled "Telemetry System for a Medical Device". The Wyborny '404 and Thompson et al. '063 patents are hereby incorporated by reference herein in their respective entireties.
Typically, telemetry systems such as those described in the above-referenced patents are employed in conjunction with an external programming/processing unit. One programmer for non-invasively programming a cardiac pacemaker is described in its various aspects in the following U.S. Patents to Hartlaub et al., each commonly assigned to the assignee of the present invention and each incorporated by reference herein in its entirety: U.S. Pat. No. 4,250,884 entitled "Apparatus For and Method Of Programming the Minimum Energy Threshold for Pacing Pulses to be Applied to a Patient's Heart"; U.S. Pat. No. 4,273,132 entitled "Digital Cardiac Pacemaker with Threshold Margin Check"; U.S. Pat. No. 4,273,133 entitled "Programmable Digital Cardiac Pacemaker with Means to Override Effects of Reed Switch Closure"; U.S. Patent No. 4,233,985 entitled "Multi-Mode Programmable Digital Cardiac Pacemaker"; and U.S. Pat. No. 4,253,466 entitled "Temporary and Permanent Programmable Digital Cardiac Pacemaker".
Aspects of the programmer that is the subject of the foregoing Hartlaub et al. patents (hereinafter "the Hartlaub programmer") are also described in U.S. Pat. No. 4,208,008 to Smith, entitled "Pacing Generator Programming Apparatus Including Error Detection Means" and in U.S. Pat. No. 4,236,524 to Powell et al., entitled "Program Testing Apparatus". The Smith '008 and Powell et al. '524 patents are also incorporated by reference herein in their entirety.
A commercially available example of a programmer used for communicating with implanted medical devices is the Model 9760, manufactured by Medtronic, Inc., Minneapolis, Minn. The Model 9760 programmer is based on an general-purpose microprocessor platform, e.g., on an Intel 80X86 microprocessor or the like, and includes a text and graphics display screen similar to that conventionally used with personal computers. The graphics display screen allows graphical depictions, for example, of real-time ECG waveforms transmitted from the implanted device, to be presented to the physician or clinician. Additionally, for pacemakers which have a Marker Channel.TM. capability, the event markers associated with various physiologic and pacing events detected by the implanted device can be superimposed upon or displayed alongside the ECG signal on the programmer's display, allowing the physician or clinician to observe the time relation between marker events and the ECG waveform. This gives the physician or clinician some degree of insight into whether the pacemaker is operating properly. However, as noted above, interpreting and understanding the Marker Channel.TM. data, even when superimposed upon an ECG waveform, can be difficult in view of the complex responses exhibited by state-of-the-art pacemakers.
The difficulties associated with interpreting, understanding, and or verifying proper implanted device response are exacerbated when the device is operable in multiple modes, e.g., when there is defined for the device a pacing algorithm for treating bradycardia (generally the function of a pacemaker), but also cardioversion and defibrillation algorithms for the treatment of tachycardia and fibrillation. In known pacemaker/cardioverter/defibrillators (PCDs), both ventricular fibrillation and ventricular tachycardia are identified and treated in addition to the identification and treatment of bradycardia. In PCDs, ventricular fibrillation and ventricular tachycardia are typically identified using complex rate-based criteria. In such devices, their operational algorithms commonly specify rate or interval ranges that characterize one or more types of ventricular tachycardias and fibrillation. Counts of the measured RR intervals which fall into the rate ranges are used to determine whether a tachycardia is present and to diagnose the particular tachyarrhythmia. The detection methodologies practiced in such devices may be difficult for the physician to follow, as the individual intervals may increment or not increment an individual count depending upon factors other than the interval duration alone. For example, rapid onset criteria based upon preceding intervals may be required to continue counting. In some devices, whether a measured R-R interval increments a count, and which count is incremented, may be a function of both the individual interval duration and the average rate over a preceding series of intervals.
In implantable tachyarrhythmia devices, each of the possible diagnoses provided by the device will trigger a predefined therapy, with the general aggressiveness of the therapies increasing from least aggressive if the diagnosis is a slow ventricular tachycardia to most aggressive if the diagnosis is ventricular fibrillation. For example, anti-tachycardia pacing may be employed in response to a diagnosis of slow ventricular tachycardia, cardioversion may be employed if the diagnosis is fast ventricular tachycardia, and defibrillation may be employed if the diagnosis is fibrillation.
Those of ordinary skill in the art will appreciate that with PCDs operable according to complex operational algorithms and in such a variety of modes in which many different device responses may be exhibited, assessment of device operation based only upon observation of the ECG, or even of the ECG in conjunction with conventional event markers, can be difficult. Moreover, a physician or clinician wishing to assess device operation based upon Marker Channel.TM. event markers may not be familiar with each and every subtle detail of the device's operational algorithm(s), or of its programming and operation, further complicating assessment of device operation.
One advancement related to assisting the physician or clinician in interpreting or assimilating the Marker Channel.TM. data provided from an implanted device is exemplified in the Elite.TM. and Elite II.TM. pacemakers, commercially available from Medtronic, Inc., Minneapolis, Minn. The Elite.TM. and Elite II.TM. pacemakers are programmable via a Medtronic Model 9760 programmer, which is capable of converting ten-second traces of Marker Channel.TM. telemetry data into so-called Marker Channel.TM. Diagrams, which appear on the programmer's graphical display and may be printed from the programmer's printer. The Marker Channel.TM. Diagram is an enhancement of the event marker data received via Marker Channel.TM. telemetry. The diagram is intended to further clarify operation of the pacemaker and simplify analysis of the pacemaker's operation and the patient's ECG. Lines and symbols are provided to represent more details of pacemaker operation. For example, the interrelation of timing intervals, including blanking and refractory intervals, can be revealed in the diagram.
Although improved uplink telemetry systems, provisions for communication of Marker Channel.TM. data to an external unit, Marker Channel.TM. diagramming capabilities and the like have alleviated some of the difficulties associated with assessing implanted device performance, it is believed that there remains a need for simplifying the task of interpreting the information provided from an implanted device.